Linear regulator is a common circuit module in integrated circuits, its basic structure is shown in FIG. 1. For example, in order to provide a stable voltage to the power MOSFET in the high speed MOSFET driver, the linear regulator is necessary to produce driving voltage.
Taking the application of the linear regulator in half-bridge MOSFET driver as an example, as shown in FIG. 2, the MOSFET has gate capacitances Cgs, Cgd with large capacitance. And they are equaled to a single gate capacitance CG. For the MOSFET driver needs to control the on and off states of the MOSFET periodically, the MOSFET driver circuits need to charge and discharge the capacitance CG quickly. With switching the state of the MOSFET driver circuit, the load of the linear regulator will abruptly change as well. Taking the low N-MOSFET driver and the corresponding linear regulator N as an example, as shown in FIG. 3, when the MOSFET is off, the output of the linear regulator is disconnected from the MOSFET. At this time, the load capacitance of the linear regulator is small and the position of the output pole is high, and it is stable for the linear regulator with an internal dominant pole. When the MOSFET is on, the output of the linear regulator connects to the gate of the MOSFET. At this time, the main load of the MOSFET driver circuit is the gate capacitance CG and the output pole decreases significantly so as to reduce the phase margin of the linear regulator, to degrade the loop performance easily and even to cause the system self-oscillation. When the state of the MOSFET driver circuit switches, the CG connects and disconnects to the MOSFET driver circuit periodically. It influences the stability and the response speed of the linear regulator without the external capacitance. The main reason for this problem is that the linear regulator without capacitance is usually designed for resistance loads or certain capacitance loads. But the MOSFET working in off and on state and its driver circuit is a capacitances load which changes periodically. Its stability, response speed and optimum frequency compensation circuit need to be concerned separately according to the two states of the loads.
This kind of problem also occurs in the condition of applying the linear regulator which capacitance load may change abruptly, especially in the linear regulator without external capacitance.